IK control bone not rotating linked bone when behind it
The movements are limited by the IK settings for the lower leg bones. That is the IK settings not the IK Constraint settings. While in pose mode select the lower leg bone and you will find an Inverse Kinematics panel under bone properties.



There you have enabled a limit and set it to 0-90 degrees for the x-axis. While this does seem like a reasonable setting, it is causing the upper bone to take the rotation when the rotation of the lower leg could be negative.